AIS Know-How: Data transfer (SOTDMA vs. CSTDMA)

Since the end of 2000, the Automatic Identification System (AIS) has become a binding standard to improve the navigation and safety at sea. Position, course, speed, and other ship data are transmitted alternately on the two maritime VHF channels 161.975 MHz and 162.025 MHz. Per minute there are 2,250 Slots available for the transmission of AIS data telegrams.

Two methods are available for transmitting the AIS data telegrams.
On the one hand, the CSTDMA (Carrier Sense Time-Division Multiple Access) method, which is used primarily for pleasure boating (Class B). The CSTDMA method uses carrier-sensing to defer transmissions until no other stations are transmitting. If a free slot is found, the transmission starts. Than the transmission is stopped for a 30 second time interval before the next one starts.
On the other hand, there is the SOTDMA (Self-Organized Time-Division Multiple Access) technology, which was used only by commercial vessels (Class A) but is now also authorized for pleasure boating at Class B level. With the SOTDMA method only the first free slot for the transmission has to be found
. The next five slots are always reserved automatically.

The following hierarchy results for the transmission of AIS data telegrams:

  1. Class A (commercial vessels) & SART (rescue transmitter)
  2. Class B SOTDMA (pleasure boating)
  3. Class B CSTDMA (pleasure boating)

 

The main advantages of the SOTDMA compared to the CSTDMA technology for pleasure boating (Class B) at a glace:

For more safety at sea - switch to SOTDMA Class B technology now!

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